Talk:Ankle brachial pressure index

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Good catch on the second reference, DR! JFW | T@lk 03:36, 14 March 2006 (UTC)

As far as I understand, the blood pressure in the ankle is measured by placing a cuff proximal to the point where the ultrasound probe listens for the return of blood flow. Since the cuff cuts off the flow, I don't see how the pressure can be increased by a 'reflected pressure wave'. Flow is restored when the blood pressure equals the cuff pressure and the pressure distal to the cuff is zero. Any explanations

—The preceding unsigned comment was added by MagnusBL (talk • contribs). 18:10, 4 October 2006
Good question, and its been too long since I last studied haemodynamics... so here goes:
  • The instant that the cuff pressure drops to be precisely that of systolic pulse pressure, there is nothing for the Doppler to hear - if the pressure is truly identical then the pressure wave just neutralises the total compression of the artery but the "force" is spent by the time it reaches the distal most part under the cuff, so no pressure is "left" for any blood flow more distally to where the Doppler probe is listening. The cuff pressure needs to drop a little further so that there is enough blood flow for the Doppler to pick up (both in terms of volume of flowing blood and some minimum speed for the Doppler to register). Hence when the Doppler "hears" flow again, there is a small pulse of flow and pressure gradient from the distal edge of the cuff and the more distal foot - and with pressure wave travelling far faster through a medium than the medium actually flows there will be some reflected pressure by the time enough blood is flowing for the probe to detect... and this reflected pressure increases the overall pressure in the region of the Doppler probe. Of course using a Doppler probe rather than a normal stethoscope gives a signal closer to the true systolic value (generally to nearest 1-2mmHg rather than 2-5mmHg)
  • It might be assumed that it is the previous divisions of the blood vessels from abdomen down to the area proximally above the cuff that is providing a "wall" for pressure waves to reflect off (aorta into 2 common iliacs, then to internal & external iliac and finally of course the multiple split behind the knees), thus impeding blood flow down to the legs (but proximal to where the cuff is placed). This is not the explanation taught to me by Raymond Gosling. If I recall correctly, divisions off a main blood vessel is not generally to an equal sum of cross sectional areas (i.e. a blood vessel that divides into two has a slightly smaller cross-sectional area than the sum of the cross-sectional areas of the two branches - hence there generally is no reflection off a simple major artery branch). see Blood flow equations for how small differences in radii has a large effect on flow resistance (but it all then gets very complex very quickly - see Navier-Stokes equations !).